1. Field of the Invention
The invention relates to an electric clock of the type which uses a frequency divider to step down oscillations from an electric oscillator.
2. Description of the Prior Art
Quartz clocks are already known which use a vibrating quartz crystal operating in the megahertz range and whose output frequency is reduced, by means of a frequency divider, to a value ranging in the Hertz range. When a stepper motor is employed for driving a hand mechanism, the output frequency of the oscillator is reduced to 0.5 to 1 Hertz, or, when a synchronous motor is used, to 50 or 60 Hertz. For reasons of economy, however, a stepper motor is usually provided as the drive means for the hand mechanism. The low-frequency signal supplied by the frequency divider is fed, in one known device, to a pulse shaper stage, wherein the duration of the individual pulses of the low-frequency signal is reduced. The pulse shaper stage is constructed as a logic circuit which is triggered by various higher frequency output signals from the frequency divider and operates in the manner of a mixer. The pulse train transmitted by the pulse shaper stage is fed to a control stage which connects the stepper motor to the operating voltage source. The stepper motor is connected to the hand mechanism for the duration of one pulse prevailing at the control stage input. The stepper motor is consequently acted upon by square wave pulses of identical or alternating polarity, depending on the structure of the control stage.
Quartz clocks are generally operated independently of an a.c. power network, and are operated by means of a battery as voltage source. Thus, the power consumption of quartz clocks must be as small as possible. This requirement could be met by feeding short duration square-wave pulses to the stepper motor. This is possible per se within certain limits, but, when the pulse duration is shortened, an increase in the natural vibrations of the motor armature occurs. This causes a quivering movement of the second hand and, even more serious, it causes relatively loud noises to be produced.
To eliminate these shortcomings, attempts have been made to dampen the natural vibrations of the motor armature mechanically. In one known device, an inertia mass, such as a ring, a sleeve, or a disc, is loosely mounted on the armature shaft. In another known device, a portion of the armature shaft extends into or through a dampening chamber filled with silicon oil. However, this and other known devices using a mechanical dampening can only incompletely suppress the natural vibrations of the motor armature. Moreover, even if a rather acceptable noise dampening and dampening of the second hand movement could be achieved, the dampening agents eventually wear out and must therefore be replaced. Finally, a determination of the optimal possible dampening in the individual embodiments is difficult and time-consuming since it is necessary to proceed empirically.